High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in communication traffic. These systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical fiber having a transparent core having a refractive index n.sub.1 surrounded by transparent cladding material having a refractive index n.sub.2 which is less than n.sub.1.
It has been known for some time that light can be propagated along a transparent fiber structure having a refractive index that is greater than that of its surroundings, and clad fibers have been employed to transmit light over relatively short distances. The numerical aperture (NA) of such a fiber, which is a measure of the light gathering ability thereof, can be approximated by: ##EQU1## where n is the average refractive index of the core and cladding and .DELTA. is the refractive index difference between the core and cladding. In conventional optical fibers .DELTA. is made quite large so that the NA is large, and therefore the fiber is capable of gathering a relatively large amount of light emitted by a source.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in fiber optic form. Various methods employing the flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Pat. Nos. 3,711,262; 3,737,292; 3,737,293 and 3,826,560, the latter patent being directed to the formation of gradient index fibers. In accordance with one embodiment of the flame hydrolysis process a plurality of constituents in vapor form are entrained in a gaseous medium in predetermined amounts and thereafter are oxidized in a flame to form a soot having a predetermined composition. The soot is applied to the surface of a rotating cylindrical starting member. After a first layer of soot is deposited to form the core glass, the composition of the soot is changed to form the cladding glass. Heretofore, the soot was either sintered as deposited, or it was sintered in a subsequent operation. After the starting member is removed, the resulting cylindrical hollow redraw blank is heated to a temperature at which the material has a low enough viscosity for drawing and is drawn to reduce the diameter thereof until the inner walls of the hollow member collapse. Continued drawing further reduces the fiber diameter until an optical waveguide fiber having the desired dimensions is formed. In accordance with another embodiment of the flame hydrolysis process the soot is deposited on the inner surface of a glass tube, and the resultant structure is drawn into a fiber.
The value of .DELTA. has been maintained relatively small in optical waveguides for a number of reasons. The cladding of low loss optical waveguides has usually been formed of a high purity glass such as silica, whereas the core has been formed of the same high purity glass to which a sufficient amount of dopant material has been added to increase the refractive index of the core to a value greater than that of the cladding. Optical waveguides initially formed by the flame hydrolysis process employed multivalent metal oxides such as titanium oxide, tantalum oxide and the like as the dopant material. An inherent problem of such waveguides was the limitation of dopant material to no more than about 15 wt.%. Although fused silica has excellent light transmission qualities in that absorption and intrinsic scattering of light thereby is exceptionally low, the addition of an excessive amount of the aforementioned multivalent metal oxide dopant material to increase the refractive index would cause absorption of light energy and intrinsic scattering of light to increase to undesirable levels.
Pure germania also has excellent light transmission qualities in that its absorption and intrinsic scattering of light is exceptionally low. Moreover, the refractive index of germania is sufficiently greater than that of silica so that germania can be employed as a dopant to increase the refractive index of silica. Initial attempts to utilize the flame hydrolysis process to form germania-doped silica fibers were unsuccessful since less than 1 wt.% of germania appeared in the fiber regardless of the amount of germanium tetrachloride supplied to the hydrolyzable mixture. It is theorized that the combustion flame temperature, combined with the overall furnace temperature of 1750.degree.-1850.degree. C. employed in those methods, is sufficiently high so that germania tends to volatilize rather than vitrify. Available vapor pressure data tends to support such a theory. However, glasses containing up to 100 wt.% high purity germania can be produced by the method disclosed in U.S. Patent application Ser. No. 321,109 entitled "Fused Oxide Type Glasses" filed Jan. 4, 1973 by P. C. Schultz. In accordance with that application, glasses having a high germania content can be produced by the flame hydrolysis process by depositing the oxide particles as a porous vitreous preform and then consolidating this porous preform to a solid nonporous body. The temperature must not exceed about 1600.degree. C. in any case and should be maintained, during consolidation, within a range varying between the minimum consolidation temperature of the glass being produced and about 200.degree. C. thereabove.
It was found that by maintaining process temperatures below those that would cause volatilization of GeO.sub.2, step type optical waveguide fibers comprising germania doped cores could be formed. However, the numerical apertures of such fibers could not exceed about 0.2 since only a limited amount of GeO.sub.2 could be incorporated into the fiber core due to a mismatch of core-cladding characteristics such as thermal coefficient of expansion and softening point temperature. Consider, for example, an attempt to fabricate by the flame hydrolysis process an optical waveguide fiber having a core comprising 20 wt.% GeO.sub.2 and 80 wt.% SiO.sub.2. The coefficient of expansion of such a core glass is about 19.times.10.sup.-7 per degree C., while the coefficient of thermal expansion of a pure fused silica cladding is about 6.times.10.sup.-7 per degree C., and the core-cladding expansion mismatch of about 13 points would probably cause the soot preform to crack during cooling after the soot consolidation process. The addition of about 13-18 wt.% B.sub.2 O.sub.3 to the SiO.sub.2 cladding glass will increase the expansion coefficient thereof to a value within acceptable limits so that preform breakage can be avoided. However, as a consequence of the addition of B.sub.2 O.sub.3 to the cladding glass, the draw temperature of the cladding glass will be between 50.degree. C. and 210.degree. C. below that of the core glass, which is about 1800.degree. C. With this kind of viscosity difference, fiber diameter control becomes a severe problem. Thus, optical waveguide fibers, wherein germania is employed as the refractive index increasing dopant material in the core, have had relatively limited .DELTA. values.
The small .DELTA. values of optical waveguides has resulted in numerical apertures that are much smaller than those of conventional optical fibers. Whereas the numerical apertures of commercial optical fibers or light pipes of the conventional type may be as high as about 0.6, the numerical apertures of optical waveguides have usually been between about 0.15 and 0.2.
Optical waveguides are often grouped into cables or bundles to provide redundancy in case of fiber breakage and to transmit a greater amount of the light generated by a source. Attenuation .gamma. due to random fiber bends, which can be caused by the cabling process, is given by the equation: ##EQU2## where c and p are parameters related to the geometry of the random bends and fiber index gradient and a is the fiber core radius. Examination of equation 2 shows that the distortion loss .gamma. can be reduced by decreasing the core radius, increasing .DELTA., or decreasing p and c by appropriate fiber packaging. The present invention has resulted from an attempt to decrease fiber loss due to random bends by increasing .DELTA.. Because of the relationship set forth in equation 1 such fibers will also exhibit an increased NA.